Heroes of the Telegraph

'This crowning marvel of the electric telegraph,' as Sir William Thomson happily expressed it, was followed by another invention in some respects even more remarkable. During the winter of 1878 Professor Bell was in England, and while lecturing at the Royal Institution, London, he conceived the idea of the photophone. It was known that crystalline selenium is a substance peculiarly sensitive to light, for when a ray strikes it an electric current passes far more easily through it than if it were kept in the dark. It therefore occurred to Professor Bell that if a telephone were connected in circuit with the current, and the ray of light falling on the selenium was eclipsed by means of the vibrations of sound, the current would undulate in keeping with the light, and the telephone would emit a corresponding note. In this way it might be literally possible 'to hear a shadow fall athwart the stillness.' He was not the first to entertain the idea, for in the summer of 1878, one 'L. F. W.,' writing from Kew on June 3 to the scientific journal NATURE describes an arrangement of the kind. To Professor Bell, in conjunction with Mr. Summer Tainter, belongs the honour of having, by dint of patient thought and labour, brought the photophone into material existence. By constructing sensitive selenium cells through which the current passed, then directing a powerful beam of light upon them, and occulting it by a rotary screen, he was able to vary the strength of the current in such a manner as to elicit musical tones from the telephone in circuit with the cells. Moreover, by reflecting the beam from a mirror upon the cells, and vibrating the mirror by the action of the voice, he was able to reproduce the spoken words in the telephone. In both cases the only connecting line between the transmitting screen or mirror and the receiving cells and telephone was the ray of light. With this apparatus, which reminds us of the invocation to Apollo in the MARTYR OF ANTIOCH--